Compared to those conceived naturally, babies that resulted from simple IVF, or in vitro fertilization — mixing eggs and sperm in a lab dish — had no greater risk of birth defects once factors such as the mom's age and smoking were taken into account.

However, birth defects were more common if treatment included injecting a single sperm into an egg, which is done in many cases these days, especially if male infertility is involved. About 10 percent of babies born this way had birth defects versus 6 percent of those conceived naturally, the study found.

It could be that the extra jostling of egg and sperm does damage. Or that other problems lurk in the genes of sperm so defective they must be forced to fertilize an egg.

“I don't want to scare people,” because the vast majority of babies are born healthy, said the study's leader, Michael Davies of the University of Adelaide in Australia.

Couples could use simple IVF without sperm injection, freeze the embryos and implant only one or two at a time, he said. All of those can cut the chance of a birth defect.

The study was published online Saturday by the New England Journal of Medicine and presented at a fertility conference in Barcelona, Spain.

Health agencies in Australia paid for the research.

In the United States, more than 60,000 babies were born in 2009 from 146,000 IVF attempts. About three-quarters of them used ICSI, or intracytoplasmic sperm injection.

ICSI was developed because of male infertility. But half the time, it was not done for that reason but to improve the odds that at least some embryos will be created from an IVF attempt. Many clinics do it in all cases.

The study used records on nearly 303,000 babies conceived naturally and 6,163 conceived with help in Australia from 1986 through 2002, plus records on birth defects detected by age 5.

Among fertility treatments, only ICSI, the sperm injection, resulted in higher rates of birth defects once other factors that affect these odds were taken into account.

Frozen embryos were less likely to result in birth defects than fresh ones used soon after they were created. Defective ones may be less likely to survive freezing and thawing, so the fittest embryos result in pregnancies, Davies said.

Babies born to women with a history of infertility who ended up conceiving on their own, or who had natural pregnancies after assisted ones, also had higher rates of birth defects. That suggests that infertility itself is playing a role.